Tinning is a technique used in soldering to prepare a metal surface for the application of more solder. We start by cleaning the tip of a soldering iron using a cleaning sponge. Brass sponges or even damp kitchen sponges as a cleaning sponge for your soldering iron, and this should be done frequently as we solder. As we solder components together, excess solder will build up on the tips of our irons. we need to tin the tip in order for just a thin coating of solder to be present. this allows for excellent heat flow, as well as prevents poor soldering connections from being created.
Many new soldering irons need to be tinned. All this means is that you need to heat up your iron and melt a bit of solder on the iron's tip. This gets the iron ready for use.
Another time you'll use tinning is when you are trying to solder two or more wires together. Because wires will conduct the heat of a soldering iron across the length of the wire, it can be challenging to solder wires together without first having a bit of solder on them.
Tin a solder pad is helpful when you want to solder a wire directly to a pad. This technique should not be done with other components as it has the potential to create bad solder joints. However it's fine for creating joints between a wire and a solder pad.
To tin the pad, hold the tip of your soldering iron against the pad you want to tin and then touch a bit of solder to the pad. The solder will melt and form a bit of a blob of solder on the pad.